Improving the reliability of multiplexed fiber optic low-coherence interferometric sensors by use of novel twin-loop network topologies.

Novel twin-loop network topologies for multiplexing fiber optic low-coherence reflectometric sensors are proposed and theoretically analyzed. The sensing fibers are made by connecting segments of standard single-mode fibers with partial reflections at the fiber joints and are completely passive. Absolute length measurement can be made for each segment of the sensing fiber so that strain or temperature distribution along the entire sensing fiber can be derived. It is expected that novel topologies would help as to improve the reliability of the sensor network by providing multiple accesses to each of the sensing segment so that most of the sensing segments can still be interrogated even when one or more point breakages occur along the transmission fibers. A nine-sensor twin-loop sensing network was constructed and experimentally tested by use of a Michelson low-coherence interferometer, and the results obtained agree with our theoretical prediction. The novel network topologies may be used for large-scale smart structure applications where breakages of transmission fibers may occur during the stages of sensor imbedding, installation, and structure-in-service cycles.